Trespasser attempted to mix realistic physics, a fully interactive environment, and a full 3D outdoor environment, all of which hadn't been attempted before. The problems were that Dreamworks wanted to rush development and the technology was too advanced for the computers of the time, forcing developers to dumb down the game just to get it halfway stable for release. The game would have been better had it been made 5-10 years later than it was, but it did inspire Valve to put realistic physics into Half-Life 2.
There is a fan-made patch for the game called the ATX patch which fixes many of the issues of Trespasser (allows you to adjust draw distance to enable the engine to render 3D models at farther distances than they render in the retail game, enables jumping velociraptors, fixes jump consistency, and recently fixes the "esc key error" on newer computers). There's also a fan-made toolset to create custom content. All of which was done without any official involvment from Dreamworks or EA Games and without any source code. It would help to have the source code but EA Games is ignoring any requests for it.